EuroFlyer wrote:
Maybe I will be chastised for that, but – why are underpowered four seater airplanes being certified in the first place ?
That is a question I have been asking myself for ever. Allmost all our planes have ridiculous payloads and basically none of the 4 seaters available are “real” four seaters, but almost all have 200-250 kg payload. I’m sorry, for todays “standard weights” that means they are two max three seaters. With the loads mentioned however, an occupancy of 4 may be possible.
I found a POH for a Robin DR400/120 here
local copy
Weight and Balance sais the following:
Empty Weight is 575 kg (Factory standard, so let’s say 600 kg for a typical airplane)
MTOW is 900 kg.
Fuel capacity is 110 liters, which means 78 kg.
Full fuel payload in this case would be between 220 and 230 kg, which is actually not bad at all, quite a few “4 seaters” with 160 or 180 hp have less. With the optional 150 liters, it would be considerably less.
For a flight around 1 hour you’d need therefore about 50-60 liters on board, which is 40 kgs. Consequently, payload would be around 270 to 290 kg, which means 4 people with an average of 70 kg. For a 120 hp plane, those are quite good values.
As a comparison: The 150 hp PA28-140/150 has a MTOW of 975 kg and an empty weight of 610 kgs. With 100 liters fuel on board you can fly around 2.2 hours and have 294 kgs available, which is in the same ballpark.
Performance:
at 4000 ft DA the plane needs 350m roll and 720 m to clear the obstacle. In summer, 4000 ft DA is quite normal, even that low down. (The POH only has sea level, 4000 ft and 8000 ft, as if anyone would try that…) At sea level with 35° C it will roll close to 300 m and use close to 600 m to get over the obstacle.
Again, that is not too far off from comparable airframes with 150 hp.
So with the load mentioned for these accidents, yes, that airplane is short on perfomance but not as much as the bare figures would show. At least not from the POH.
I have been using a DR400-120 in a Cannes → Propriano trip at ~850kg (My wife, me and child + full luggage), climbing FL70 in end of summer (28th August), and although it was not a super climber, it flew well but it was a straight flight over the see, wind calm (not much than 5kn) and high density altitude and it took ages to climb at cruise altitude.
For 1 hour, you don’t need 60 liter of avgas. Actually at least you need 1h30 fuel time, which is 30~35 liter (22l/h). I flew 2 hours for my trip and planned 3h (65 liters). At that load, I touched down at LFKO on the R27 and didn’t brake too hard to take the taxiway which is roughly 250m after threshold.
The DR400-120 is a good small plane at low altitude, but for sure not in the moutain, and even less with pax.
While we are at it can you make PC12 NGX that is certified as 9 seats a 4 seat? as UL with full fuel on PC12 NGX (900lbs) is less than C182…
Assuming there is a POH (approved by some certifying agency) which gives perf numbers versus temperature, one cannot blame the aircraft for being crashed due to excessive loading.
Ibra wrote:
While we are at it can you make PC12 NGX that is certified as 9 seats a 4 seat? as UL with full fuel on PC12 NGX (900lbs) is less than C182…
I think you can, as well as making a plane as a higher landing classe (A to B or higher), by national regulator. The operator can also restrict it. In our club, the 120cv are limited to 2 pax (only 1 behind the front seats).
While I am at it, here are the POH performances of the beast:
Take off
Climb
Landing
So let’s say you restrict it two by regulation, now you can take off in even shorter length, or take off when the grass is even longer, or the hill bigger… Or can you ? ;-)
So how does that help?
FWIW my single really close call in 20 years of flying was due to take-off performance on a SEP out of a soft field with the proverbial 15-ft tree at the end of the runway. I believe our wheels cleared the tree by less than 1m. This was NA and DA was relatively high at around 4000ft. Erroneous weight, DA, unknown light tailwind and soft grass compounded to put us in danger.
So for me, T/O performance is not to be taken lightly. Most of the times the main issue is the distance to 50ft, but on that one occasion, it got compounded with ground run.
Normally I do not mind a short take-off run, but I do look for a clear climbing path beyond the runway or else err on the cautious side. At reasonable density altitudes, airplanes will normally climb when at MTOW (and even higher) but a very careful technique is required balancing your flight between stall, climbing, and trying to reach Vx and preferably Vy as soon as possible after take.off.
Now I fly a turbocharged aircraft which is less affected, but nonetheless have endured two high-temperature related situations:
I once took off rather heavy from LPSR at 40C and I chose the 5kt-downwind runway taking off towards the river below runway level, rather than the upwind runway towards a 50-ft high (plus vehicles) highway bridge. In the end there was performance aplenty but memories of my previous incident weighted heavily. I really like to accelerate quickly and then climb at Vy if I can. If there are obstacles, POH climb speed in our P210 is way under Vy. I avoid that as much as possible.
The other occasion was on late Jun 2019 (likely on @Ibra’s hot day of the year ) from LFMA also at 45C, again heavy with 6POB but runway is long and it was not a performance problem, but one of our iPad’s used for charting decided to quit until we were at altitude. GTN750 seems to be more tolerant of high temps. This can be an issue if you are reliant on tablets for your navigation.
There is also the question of the accuracy of POH data.
I have found the aircraft I have flown to be pretty accurate. However, the time to find out is not when you are performing a tight take-off: you can verify numbers on a normal take-off, and expect similar performance on a shorter runway: the aircraft will not know how long the runway is, only weight, surface, slope and environmental conditions.I also use a take-off performance computer I bought in the US and have found it to be very accurate too.
Interestingly this computer also provides expected ROC at Vy for NA aircraft. If you get a zero, it does not matter how long your runway is : you will not make it.
greg_mp wrote:
While I am at it, here are the POH performances of the beast:
Archacon is sea-level and 1.5km long, would you takeoff 4pob in 120CV Robin at MTOW in hot day? I see no reason why not? would I do it at 6000ft DA, absolutely NO even if it’s “ok for POH”
Now it could be that engine is only delivering 50HP that day? or about to quit on one remaining mag? but anyway once you put WOT, POH numbers goes to the bin and you have to figure out yourself, I think that aircraft should be at Vy by the time one pass 700m
